# std::lock

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< cpp‎ | thread

Defined in header `<mutex>` |  |
template< class Lockable1, class Lockable2, class... LockableN > void lock( Lockable1& lock1, Lockable2& lock2, LockableN&... lockn ); |  | (since C++11)

Locks the given Lockable objects lock1, lock2, `...`, lockn using a deadlock avoidance algorithm to avoid deadlock.

The objects are locked by an unspecified series of calls to `lock`, `try_lock`, and `unlock`. If a call to `lock` or `unlock` results in an exception, `unlock` is called for any locked objects before rethrowing.

### Parameters

lock1, lock2, ... , lockn | - | the Lockable objects to lock

### Return value

(none)

### Notes

Boost provides a version of this function that takes a sequence of Lockable objects defined by a pair of iterators.

`std::scoped_lock` offers a RAII wrapper for this function, and is generally preferred to a naked call to `std::lock`.

### Example

The following example uses `std::lock` to lock pairs of mutexes without deadlock.

Run this code

    #include <chrono>
    #include <functional>
    #include <iostream>
    #include <mutex>
    #include <string>
    #include <thread>
    #include <vector>

    struct Employee
    {
        Employee(std::string id) : id(id) {}
        std::string id;
        std::vector<std::string> lunch_partners;
        std::mutex m;
        std::string output() const
        {
            std::string ret = "Employee " + id + " has lunch partners: ";
            for (auto n{lunch_partners.size()}; const auto& partner : lunch_partners)
                ret += partner + (--n ? ", " : "");
            return ret;
        }
    };

    void send_mail(Employee&, Employee&)
    {
        // Simulate a time-consuming messaging operation
        std::this_thread::sleep_for(std::chrono::milliseconds(696));
    }

    void assign_lunch_partner(Employee& e1, Employee& e2)
    {
        static std::mutex io_mutex;
        {
            std::lock_guard<std::mutex> lk(io_mutex);
            std::cout << e1.id << " and " << e2.id << " are waiting for locks" << std::endl;
        }

        // Use std::lock to acquire two locks without worrying about
        // other calls to assign_lunch_partner deadlocking us
        {
            std::lock(e1.m, e2.m);
            std::lock_guard<std::mutex> lk1(e1.m, std::adopt_lock);
            std::lock_guard<std::mutex> lk2(e2.m, std::adopt_lock);
        // Equivalent code (if unique_locks are needed, e.g. for condition variables)
        //  std::unique_lock<std::mutex> lk1(e1.m, std::defer_lock);
        //  std::unique_lock<std::mutex> lk2(e2.m, std::defer_lock);
        //  std::lock(lk1, lk2);
        // Superior solution available in C++17
        //  std::scoped_lock lk(e1.m, e2.m);
            {
                std::lock_guard<std::mutex> lk(io_mutex);
                std::cout << e1.id << " and " << e2.id << " got locks" << std::endl;
            }
            e1.lunch_partners.push_back(e2.id);
            e2.lunch_partners.push_back(e1.id);
        }
        send_mail(e1, e2);
        send_mail(e2, e1);
    }

    int main()
    {
        Employee alice("Alice"), bob("Bob"), christina("Christina"), dave("Dave");

        // Assign in parallel threads because mailing users about lunch assignments
        // takes a long time
        std::vector<std::thread> threads;
        threads.emplace_back(assign_lunch_partner, std::ref(alice), std::ref(bob));
        threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(bob));
        threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(alice));
        threads.emplace_back(assign_lunch_partner, std::ref(dave), std::ref(bob));

        for (auto& thread : threads)
            thread.join();

        std::cout << alice.output() << '\n'
                  << bob.output() << '\n'
                  << christina.output() << '\n'
                  << dave.output() << '\n';
    }

Possible output:

    Alice and Bob are waiting for locks
    Alice and Bob got locks
    Christina and Bob are waiting for locks
    Christina and Bob got locks
    Christina and Alice are waiting for locks
    Dave and Bob are waiting for locks
    Dave and Bob got locks
    Christina and Alice got locks
    Employee Alice has lunch partners: Bob, Christina
    Employee Bob has lunch partners: Alice, Christina, Dave
    Employee Christina has lunch partners: Bob, Alice
    Employee Dave has lunch partners: Bob

### See also

unique_lock (C++11) | implements movable mutex ownership wrapper (class template)
try_lock (C++11) | attempts to obtain ownership of mutexes via repeated calls to `try_lock` (function template)
scoped_lock (C++17) | deadlock-avoiding RAII wrapper for multiple mutexes (class template)

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